1. Field of the Invention
The present invention relates to an ink jet printer that produces a multi-value output of a multi-tone image, such as a photographic image.
2. Discussion of the Background
An ink jet printer generally spouts a specific ink from ink jet nozzles on a printing medium to form small dots to execute printing. In a concrete procedure, the following procedure is repeated to execute the printing: executing dot printing while driving in a main scanning direction a nozzle array having a plurality of nozzles arranged in a sub-scanning direction; feeding a paper sheet at a predetermined pitch in the sub-scanning direction; and executing the dot printing again while driving the nozzle array in the main scanning direction.
Print outputs made by the ink jet printer are not restricted to the conventional print of letters, but it is required to print a multi-tone image, such as a photographic image, with a high quality. The ink jet printer has been improved to satisfy such a requirement, thereby attaining higher resolution and enabling the printing with the finer dots. According to a generally used method of producing a multi-value output of a multi-tone image, driving frequency of the ink jet nozzles in the main scanning direction is made approximately twice an ordinary frequency while the driving distance is minutely regulated so as to change the pixel density.
FIG. 1 shows the concept of a conventional multi-value output technique. This example shows dot formation by three-value outputs, based on print image data including four-value tone information. The four-value tone information requires at least 2-bit, and in the example of FIG. 1(a), 8-bit (b7-b0) raster byte data constitutes print image data of four pixels. Two-bit combinations for expressing each pixel are (b7,b6), (b5,b4), (b3,b2), and (b1,b0) as shown in FIG. 1(b). The 2-bit representing the tone of one pixel expresses three value outputs by assigning the value `00` to no output of dots, `01` and `10` to outputs of one dot, and `11` to an output of adjoining two dots.
In the above conventional ink jet printer, it is required to drive the ink jet nozzles at a driving frequency that is twice an ordinary frequency in order to carry out the multi-value outputs if the main scanning speed is fixed. This requires the higher-speed head driving mechanism, which undesirably increases the required cost. It may be possible to maintain the driving frequency of the head while halving the main scanning speed only in the case of the multi-value outputs. This, however, lowers the throughput of printing to one half and increases the control conditions on the main scanning speed.
Some conventional ink jet printers adopt a printing scheme of fixed-pitch sub-scans, in order to attain high-quality printing. This printing scheme controls the pitch of sheet feed in the sub-scanning direction to be a constant value so that adjoining lines in the sub-scanning direction are formed by the dots spouted from different ink jet nozzles (see U.S. Pat. No. 4,198,642). When sheet feed errors are accumulated under the minute sheet feeding control requirement, the above multi-value outputs tend to cause banding.
The nozzle pitch has been narrowed to enhance the printing resolution, but there is a manufacture limit to simply narrow the nozzle pitch. Accordingly, print heads as shown in FIG. 2 are commercially available, in which plural columns (two columns in this example) of nozzle arrays are arranged apart from each other in the sub-scanning direction to apparently narrow the nozzle pitch (k pitch in the illustrated example). In such conventional print heads, banding due to a positional displacement of the nozzles easily occurs if the head is inclined. The wider distance between the adjoining columns of the nozzle arrays makes the banding (that is, the streak-like pattern formed along the sub-scanning direction) more conspicuous.
In the conventional multi-value output technique, dots are formed consecutively in the transverse direction in the case of three-value outputs. The dot shape accordingly tends to be long from side to side as shown in FIG. 1(b). This lowers the image quality due to granularity deterioration, and requires more accurate sheet feed control because the dots do not extend in the vertical direction.